This invention relates to a signal defect compensator which may be used in a system for reproducing recorded television image information and, more particularly, to a signal defect compensator having a plurality of operating modes which are selected automatically according to the form of a particular defect.
In the reproduction of television video signals from information recorded, for example, on magnetic tape, magnetic discs or other types of recording media, a decrease or loss of the recorded high frequency signal information may be encountered. A form of defect known as a dropout may occur due to imperfections in the tape or other medium used for the video recording or due to the subsidiary equipment used in the video recorder. When the dropout in the playback signal occurs, it causes a disturbance in the reproduced video signal which appears usually in the form of random black and white streaks or flashes on the screen of the television receiver or other reproducer.
Another form of signal defect known as switching transients may also occur, particularly in a multiple head video tape recorder (VTR), as the multiple heads are sequentially switched during playback of the recorded information.
It is known that television image information is to some extent redundant from line to line. Dropout compensators (DOC) therefore generally have operated on the principle that since the information is redundant from line to line, it is possible to compensate for dropout by substituting information from a previous image line. Known defect compensators therefore store image information from a previous line and insert the previous line of stored information into the video signal when a signal dropout occurs. However, such compensators which generally rely on a loss of signal for activation will not operate in response to a switching transient defect.
In a multi-head VTR it is common practice, in the recording mode, to have two recording heads in contact with the tape simultaneously; one head just completing its excursion across the tape, and the other just beginning its excursion. During a brief interval, the two heads record redundant information; e.g., in a quadruplex VTR, the overlap is of the order of two or three television lines.
During playback, timing pulses derived from the replayed video and from a pulse generator mounted on the headwheel motor shaft are combined to cause the recorder to switch from one head to another during the replay of the redundant information. This switching action is timed to take place at some point outside the active picture information. Typically, it is located at a point during the front porch of the horizontal blanking interval in an attempt to avoid interference with the synchronizing pulse and the burst signal which is located on the back porch of the horizontal blanking interval signal. Since the switching operation occurs in the frequency modulated (FM) domain, it is substantially impossible to have the FM carriers from the two heads exactly in-phase, and the switching point represents a sharp phase discontinuity in the signal output from the headwheel equivalent to an infinite change in frequency of the FM signal. During demodulation of the signal, this discontinuity will appear as a very large narrow spike occurring during the front porch of the horizontal blanking interval.
In general, the practice in the prior art has been to suppress the switching transient by clamping the signal level during the duration of the front porch. The clamping is usually done prior to any de-emphasis networks used in the playback apparatus since the de-emphasis networks, which are essentially a low-pass filter, transform the energy in the switching transient from a tall, narrow spike to a short, flat disturbance. This disturbance can extend from the front porch into the leading edge of sync distorting the timing of those sync pulses occurring just after the head switching takes place. In a quadruplex VTR, the result in the picture is a left to right "pull" of every 16th television picture line.
Clamping of the switching transient prior to de-emphasis to avoid sync distortion results in yet another set of difficulties in signal processing. In the pre-emphasized signal, the clamping pulse produces a "notch" especially when the right edge of the picture is white. The resulting de-emphasis of this "notch" produces disturbances in sync timing which are almost as objectionable as those produced by the signal switching transient. A further difficulty exists in any method used to clamp the switching transients because although the front porch signal duration is specified; e.g., NTSC standards, and the so-called narrow front porch is out of specification, such narrow front porches occur very often in tape recordings. A recorded video picture signal having such a narrow front porch will have the "notch" so close to sync that the difference between white right edge and black right edge pictures will produce non-identical disturbances in sync timing. The result is a "pull" in the picture which follows the motion of any white object near the right side of the picture.